Cloning without stem cells works
US scientists say stem cells are not necessary for cloning and other cells may even be better candidates.
The Pittsburgh University team created two baby mice from a fully matured blood cell that itself is incapable of making more of its own kind.
Experts had thought only immature stem cells, which can become many types of other cell, were capable of doing this.
The authors told Nature Genetics it may actually have been mature cells that made Dolly the sheep, not stem cells.
A UK expert said the findings overturned the conventional wisdom that the less mature a cell, the more useful it was likely to be for cloning.
Alternative routes
Somatic cell nuclear transfer (SCNT) - the scientific term for cloning - is creating an embryo by taking the nucleus, which houses the genetic material of a cell, from one cell and putting it into an unfertilised egg that has had its own genetic material removed.
The resulting embryo is then an exact genetic copy of the cell from the animal or person that donated the nucleus.
Stem cells are still at an early stage of development, and retain the potential to turn into many different types of cell that make up tissues and organs, which is why experts have heralded their promise for treating a variety of genetic diseases.
But experiments using adult stem cells taken from mature tissue to make early stage embryos have yielded disappointing results, with success rates of 1-5%.
Dr Tao Cheng and colleagues tested whether a fully matured type of white blood cell, called a granulocyte, could propagate early embryos.
Not only was this successful, the granulocyte was far better at this than its immature ancestor cells destined to become granulocytes.
Challenging convention
Between 35% and 39% of the mature granulocytes yielded early stage embryos called blastocysts.
In comparison, only 4% of the immature stem cells could produce blastocysts.
And only the mature, or "differentiated", granulocytes were able to produce two live cloned mice pups, although both of these died within a few hours of birth.
To check the success of the findings, the scientists also tried cloning using embryonic stem cells - cells taken from blastocysts, rather than using mature tissue to find the donor cells.
Nearly 50% of these yielded blastocysts and 18 cloned pups were born.
However, the use of embryonic stem cells is controversial because opponents argue that all embryos, whether created in the lab or not, have the potential to go on to become a fully fledged human, and as such it is morally wrong to experiment on them.
Scientists have been looking for successful alternatives to embryonic stem cells.
Dr Cheng said the results clearly showed there was no advantage in using adult stem cells over mature fully differentiated cells.
"We can say with near certainty that a fully differentiated cell such as a granulocyte retains the genetic capacity for becoming like a seed that can give rise to all cell types necessary for the development of an entire organism."
Based on his findings, Dr Cheng said it was hard to believe that Dolly the sheep could have possibly been derived from adult stem cells.
Stem cell expert Dr Stephen Minger, from King's College London, said: "The findings are quite surprising.
"Up until this the conventional wisdom was that the less mature a cell, the more likely it is to be reprogrammed. This work suggests the contrary.
"Certain types of mature cells could be much easier to reprogramme than expected.
"But of course this has only been done in mice and there could be a big difference when it comes to humans. It would be interesting to see whether the same is found with human cells."
